1. Field of the Invention
The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device in which the overflow of the carriers is reduced.
2. Description of the Prior Art
In recent years, the development of semiconductor laser devices having advantages such as being of a small-size, having a high conversion efficiency, and available at a moderate price has advanced the application of lasers to industrial machines and household machines.
Semiconductor laser devices have been applied in a number of fields such as optical communication and a light source for reading data from an optical disk. It is expected that improvement in properties of semiconductor laser devices will contribute to further enlargement in the range of their application. For example, as a light source for writing data on an optical disk, a semiconductor laser which operates with higher output, oscillates at a shorter wavelength and thus enables higher recording density is required.
FIG. 3 shows an example of a conventional semiconductor laser device 300 of a gain waveguide type using an AlGaInP crystal. The semiconductor laser device 300 has a separate confinement heterostructure including an active layer 35 with a quantum well structure.
The semiconductor laser device 300 has an n-type GaAs substrate 31, an n-type Ga.sub.0.51 In.sub.0.49 P buffer layer 32, an n-type Al.sub.0.51 In.sub.0.49 P cladding layer 33, and an undoped (Al.sub.0.6 Ga.sub.0.4).sub.0.51 In.sub.0.49 side barrier layer 34, a Ga.sub.0.51 In.sub.0.49 P quantum well active layer 35, an undoped (Al.sub.0.6 Ga.sub.0.4).sub.0.51 In.sub.0.49 P side barrier layer 37, a p-type Al.sub.0.51 In.sub.0.49 P cladding layer 38, a p-type Ga.sub.0.51 In.sub.0.49 P intermediate layer 39, and a p-type GaAs contact layer 310, which are layered in this order from the lowest side. Each crystal layer is successively grown by molecular beam epitaxy. The semiconductor laser device 300 also has a silicon nitride film 313, an Au-Ge-Ni electrode 311 and an Au-Ge-Ni electrode 312.
FIG. 4a shows an energy band structure in the case where a forward bias is applied to the semiconductor laser device 300 so as to inject into the active layer 35 a current of the order of a threshold value for laser oscillation. The difference in energy between the active layer 35 and the side barrier layer 37 is not satisfactory for reducing the overflow of the carriers, especially electrons injected into the active layer 35. Under the condition that carriers are excessively injected into the active layer 35, electrons are leaked out of the active layer 35, as illustrated in FIG. 4b, causing an increase in the threshold current.
In order to reduce the overflow of the electrons from the active layer 35, it is required to change the composition of the side barrier layer 37 so as to increase the difference in energy of the conduction band between the active layer 35 and the side barrier layer 37. However, energy difference cannot be made larger than a limited value. For example, in the case of a Ga.sub.0.51 In.sub.0.49 P/(Al.sub.x Ga.sub.1-x).sub.0.51 In.sub.0.49 P heretojunction, the maximum difference in energy at x=0.7 is about 0.15 eV. Another way to reduce the electron overflow is to dope the side barrier layer 37, thereby increasing the energy level of the valence band thereof. A closely related study is described in Extended Abstracts of the 22nd (1990 International) Conference on Solid State Devices and Materials, Sendai, pp. 565-568. FIG. 5 shows an energy band structure in the case where the side barrier layers 34 and 37 are doped in the semiconductor laser device 300 of FIG. 3. As seen from FIG. 5, a potential barrier to the electrons in the active layer 35 can be substantially made higher. More specifically, by doping the side barrier layer 37, a reverse potential gradient is formed against the leaked electrons in the portion of the side barrier layer 37 in the vicinity of the active layer 35 whereby reducing the overflow of the electrons.
FIG. 6 is a view showing the height of the potential barrier to the electrons, in which .DELTA. Ec is the original energy difference of the conduction band and .delta.Ec is the energy difference generated by doping the side barrier layer. The height of the potential barrier is determined by the bandgap energy Egb and the quasi-Fermi level E.sub.FVb of the p-doped side barrier layer. In FIG. 6, the quasi-Fermi level E.sub.FVb is expressed as an increase in a potential with respect to the holes, the top of the valence band being the standard. The greater the bandgap energy Egb becomes, and the further the quasi-Fermi level E.sub.FVb enters into the valence band, the greater the energy difference .delta.Ec becomes.
However, since the effective mass of the hole is usually larger than that of the electron, the quasi-Fermi level E.sub.FVb of the holes even at the increased carrier concentration, does not easily enter into the valence band as the quasi-Fermi level E.sub.FVb of the electrons enter into the conduction band. FIG. 7 shows the difference with respect to the carrier concentration between the quasi-Fermi level of the holes and that of the electrons in case of GaAs, and the similar tendency is generally observed in the other semiconductor materials. Accordingly, the potential barrier cannot be easily made high by doping the side barrier layer. In addition, when the composition itself of the side barrier layer is changed so as to increase the bandgap energy Egb, for example, the ratio of Al is increased in an AlGaAs mixed crystal on an AlGaInP mixed crystal, the bandgap energy Egb reaches the maximum at the point where the energy difference of the indirect transition becomes smaller than that of the direct transition, and cannot be made larger anymore even by further changing the composition.